"Wrongful termination" gets used loosely to describe any firing that feels unfair — but the term has a specific legal meaning, and most at-will employees can actually be terminated for reasons that feel arbitrary without it being legally wrongful.
At-will employment is the default, and it's broad
In most states, employment is presumed at-will, meaning either side can end it at any time, for almost any reason, or no reason at all — a firing that seems unfair isn't automatically illegal just because it lacks a good business justification.
What actually makes it wrongful
Termination becomes legally wrongful when it's based on a protected characteristic (discrimination), in retaliation for a protected activity (reporting harassment, whistleblowing, taking legally protected leave), or in violation of an existing employment contract.
Timing and documentation matter enormously
A termination that closely follows a protected activity — filing a complaint, requesting leave — invites scrutiny over whether the timing was retaliatory, which is why employers should document performance issues well before any termination, and employees should keep records of protected activity and its timing.
For employers: getting the documentation right protects you
A termination supported by clear, contemporaneous documentation of performance or conduct issues — not created after the fact — is far more defensible than one relying on the manager's memory of why the decision was made.